The development of highly linear Distributed Feedback ("DFB") lasers has revolutionized the delivery of analog cable television ("CATV") signals. A single lightwave link can now replace a coaxial cable supertrunk containing 20 or more cascaded broadband RF amplifiers. In addition to reducing costs and improving reliability for conventional unidirectional broadcast systems, this technology is largely responsible for the rapid acceptance of hybrid fiber/coax (HFC) as the current domestic delivery system of choice for both analog broadcast and bidirectional digital switched services.
With the analog broadcast systems, however, the stringent carrier-to-noise ratio (CNR) and carrier-to-nonlinear-distortion ratio requirements for such systems limit the number of optical fibers which can be driven by a single DFB laser transmitter, and as well, the number of subscribers or end users that can be served by such a single laser. While it is possible to improve the CNR for a given laser output power by increasing the optical modulation depth per RF subcarrier (thereby providing greater signal power relative to a noise level), such an increase in modulation depth is usually accompanied by increasing levels of nonlinear distortion.
It will be understood that an increase in CNR, without a corresponding degradation in signal quality due to increased non-linear distortion, would enable a greater number of users to be served by such a lightwave system on a per-laser basis. Accordingly, an achievement of an increased CNR while maintaining non-linear distortion essentially fixed would provide an increase in relative efficiency for such a lightwave system.